Use proper uppx scaling when either of scaling the data to the x-domain
index-range or when the uppx is < 1 (now that we support it) such that
both the fast and slow chart always appropriately scale and offset to
the y-axis with the last datum graphic just adjacent to the order line
arrow markers.
Further this fixes the `.index_step()` calc to use the "earliest" 16
values to compute the expected sample step diff since the last set often
contained gaps due to start up race conditions and generated
unexpected/incorrect output.
Further this drops the `.curve_width_pxs()` method and replaces it with
`.px_width()`, taken from the graphics object API and instead returns
the pixel account for the whole view width instead of the
x-domain-data-range within the view.
We want the fast and slow chart to behave the same on calls to
`Viz.default_view()` so adjust the offset calc to make both work:
- just offset by the line len regardless of step / uppx
- add back the `should_line: bool` output from `render_bar_items()` (and
use it to set a new `ds_allowed: bool` guard variable) so that we can
bypass calling the m4 downsampler unless the bars have been switched
to the interpolation line graphic (which we normally required before
any downsampling of OHLC graphics data).
Further, this drops use of the `use_vr: bool` flag from all rendering
since we pretty much always use it by default.
Mainly it was the global (should we )increment logic that needs to be
independent for the fast vs. slow chart such that the slow isn't
update-shifted by the fast and vice versa. We do this using a new
`'i_last_slow'` key in the `DisplayState.globalz: dict` which is
singleton for each sample-rate-specific chart and works for both time
and array indexing.
Also, we drop some old commented `graphics.draw_last_datum()` code that
never ended up being needed again inside the coordinate cache reset
bloc.
Tyler Goodlet